1. Field of the Invention
This invention relates to a composite structure and method of making composite structures. More particularly, the invention relates to composite structures which are especially adapted for use as spring and damper units in a variety of applications.
2. Description of Related Art
In the past, a number of structural composite articles have been developed using a variety of different technologies. One prior art technique involves interweaving yarns of polyester fibers with yarns of glass fibers to form a composite fabric. The fabric is then fitted within a mold and soaked with resin. The formed fabric and resin composition is then allowed to harden within the mold. While providing a generally rigid structure, such a technique includes a number of limitations, among them the inability to produce a variety of different shapes of substantial bulk or complex dimensions.
Another prior art technique involves manufacturing a light weight energy-absorbing structure made up of a laminate including a plurality of fabrics. The fabrics are pre-impregnated with a thermosetting resin and abutted against a core of cellular foam which has had a bonding material applied at the interface between the impregnated fabric and the foam. Heat is applied to cure the resin and the bonding material. However, when bonding material is used, it is often the case in hostile environments that the bonding material breaks down, an undesirable result when such structures are used in high load or stress applications.
Yet still another prior art technique involves the manufacture of a thermo-formable composite article comprised of a core layer made of a synthetic resin foam. A pair of adhesive layers are placed in contact with the opposite upper and lower surfaces of the core layer and a pair of fabric layers are superimposed on the opposite upper and lower surfaces of the core layer. The fabric layers are bonded to the core layer by the adhesive layers. At least one of the fabric layers is made of a non-woven fabric which has been impregnated with a thermoplastic or thermosetting resin. This technique also involves the disadvantages noted above with respect to the use of adhesives.
In boat building, especially glass fiber boats, hulls are typically reinforced by a grid arrangement of structural members known as "stringers" and "bulkheads," hereafter collectively referred to as "stringers." Present boat-building technology typically employs plywood stringers, although a growing percentage uses glass fiber/foam stringers. The plywood stringer system involves several process stages. The plywood is first cut to shape by a cutter or by hand with the use of templates. The stringer system which is commonly made up of interlocking stringers and bulkheads is assembled outside of the boat, with interlocking pieces being held with staples. Once the system is assembled, it is then placed in a boat as a module and laminated into place using a combination of reinforcing fabrics.
Glass fiber stringers are installed in a slightly different manner from plywood stringers. One way of installing such stringers is to cut blocks of foam into the shape of the stringers. The stringer shaped foam is then bonded into the hull and covered with glass fiber. Another way is that the glass fiber shell of the stringer is first laminated using a male mold. The stringer is then ground to fit into the boat and once the stringer has been ground to fit, it is spaced off of the bottom of the boat to avoid cracking problems. The stringer is then laminated into place and holes are drilled into the stringer and the stringer is filled with a two-part urethane foam.
Future boat construction is expected to use closed molding technologies. The reason for this is 1) to control noxious vapor emissions into the environment, 2) to reduce labor costs and (3) to create structurally superior constructions. Such future technology will generally require use of stringer systems made of composite structures compatible with such closed moldings. Conventional stringer systems are not well suited to such technologies since they will need to be installed after the hull has been laminated using an open mold. Wood stringers would be difficult to incorporate into a closed mold due to height and width constraints. Further, prepositioning of glass fiber reinforcement normally hand laminated over the wood or foam stringer system would be costly and create quality control problems. Conventional glass fiber stringers also pose problems due to the void in the stringer structures. Specifically, since foam is injected into such stringer after installation, it becomes difficult to keep the resin from filling the inside of the stringer.
Another problem which arises in fabrication of composite structures is the difficulty in constructing a composite member which provides shock absorbing characteristics and resiliency. Such components can be used in varied applications such as shock isolation members between a boat hull liner or deck and the outer hull, engine mounting systems, leaf springs and in any other type of composite structure which requires an engineered resiliency and/or shock absorption. In the past, these characteristics have been provided using shock absorbing rubber mounts, conventional metal or plastic spring and hydraulic shock absorbers. However, such systems have proven unsuitable for use in certain composite structures for a variety of reasons. In some instances, environmental conditions dictate that conventional spring and damping systems not be used due to the corrosive effect of salt water for example. Further, the point of attachment between a composite structure and a non-composite spring/damping system will typically result in a high localized stress concentration which can tend to damage to the composite structure.
In accordance with the invention, these and other problems with the prior art are avoided by providing a composite structure and method of making such structure which avoids the use of adhesive and lends itself to a variety of shapes, structures and applications. In one specific aspect, the composite articles manufactured in accordance with the method are used as spring and damper systems for composite constructions.